This invention relates generally to gas turbine combustors and more particularly concerns combustion liners having a catalytically active thermal barrier coating with vortex generating ridges formed on the inner surface thereof.
Traditional gas turbine combustors use diffusion (i.e., nonpremixed) flames in which fuel and air enter the combustion chamber separately. The process of mixing and burning produces flame temperatures exceeding 3900.degree. F. Since the maximum temperature conventional combustor liners are generally capable of withstanding is on the order of about 1500.degree. F. steps to protect the liners must be taken. This is typically done by film-cooling which involves introducing the relatively cool compressor air into a plenum surrounding the outside of the liner. The air from the plenum passes through louvers in the liner and then passes as a film over the inner surface of the liner, thereby maintaining liner integrity.
Because diatomic nitrogen rapidly disassociates at temperatures exceeding about 3000.degree. F., the high temperatures of diffusion combustion result in relatively large NO.sub.x emissions. One approach to reducing NO.sub.x emissions is to premix the maximum possible amount of compressor air with fuel. The resulting lean premixed combustion produces cooler flame temperatures and thus lower NO.sub.x emissions. Although lean premixed combustion is cooler than diffusion combustion, the flame temperature is still too hot for conventional liners to withstand. Furthermore, because the advanced combustors premix the maximum possible amount of air with the fuel for NO.sub.x reduction little or no cooling air is available making film-cooling of the liner impossible. Thus, a thermal barrier coating in conjunction with "backside" cooling have been considered to protect the liner. Backside cooling involves passing the compressor air over the outer surface of the liner prior to premixing the air with the fuel.
Lean premixed combustion reduces NO.sub.x emissions by producing lower flame temperatures. However, the lower temperatures, particularly along the inner surface or wall of the liner, tend to quench oxidation of carbon monoxide and unburned hydrocarbons and lead to unacceptable emissions of these species. To oxidize carbon monoxide and unburned hydrocarbons, a liner would require a thermal barrier coating of extreme thickness (50-100 mils) so that the surface temperature could be high enough to ensure complete burnout of carbon monoxide and unburned hydrocarbons. This would be approximately 1800.degree.-20000.degree. F. for combustors of typical lengths and flow conditions. However, such thicknesses and temperatures are beyond materials capabilities. Thermal barrier coatings degrade in unacceptably short times at these temperatures and such thick coatings are susceptible to spallation.
Accordingly, there is a need for a combustion liner which can withstand combustion temperatures without film-cooling and yet maintain flame stability and burn out carbon monoxide and unburned hydrocarbons.